1. Field of the Invention
This invention concerns turbo-compressors driven by the exhaust gases of internal combustion engines. It concerns, in particular, the turbine housings of turbo-compressors.
2. Description of the Related Art
It is conventional for a turbo-compressor equipping an internal combustion engine to consist of two turboshaft engines, one driving (the gas turbine) and the other being driven (the air compressor), which are connected to opposite ends of a common shaft. The exhaust gases of the internal combustion engine, or at least a portion of those gases, are sent to the turbine so that the energy of the gases rotates the common shaft and the compressor wheel, which produces suction and compression of the ambient air, the latter then being sent into the intake manifold of the internal combustion engine. This arrangement of a turbine and a compressor is well known and is similar to a supercharger, except that in the case of the latter, the compressor is rotated by a direct mechanical link with the crankshaft.
The generally centripetal turbine of the turbo-compressor consists of a spiral-shaped inlet casing, which radially distributes the exhaust gases around a wheel, and of a cylinder-shaped outlet which recovers the gases in the axis of the wheel. wheel. The stationary part of the turbine consists of a housing containing cast iron inlet and outlet pipes. The gas inlet, extending perpendicular to the axis of rotation of the wheel, comprises a flange intended to be bolted at the outlet of the exhaust manifold, while the outlet, on an extension of the axis of rotation of the wheel, comprises a flange designed for connection to the exhaust pipe of the vehicle. A turbo-compressor is thus known according to document EP-A-342361.
The use of a turbo-compressor offers numerous advantages, notably in that it improves filling of the internal combustion engine by increasing the density of the air and makes it possible, therefore, to widen the gear ratios and to lower the volume capacity of the engine for a given power. However, the use of a turbo-compressor also presents a number of disadvantages, notably, that of slowing down the temperature rise of the catalytic converter in the exhaust line at the turbine outlet.
In fact, the pollution standards for motor vehicles equipped with internal combustion engines are, in general, becoming more and more strict in all industrialized countries. The automobile industry is therefore totally mobilized to find technical solutions for responding to these constraints and has adopted the use of exhaust devices treating the noxious components of exhaust gases by catalytic conversion.
These purification devices, also called catalytic converters, make possible the oxidation of unburnt hydrocarbons and of carbon monoxide as well as the reduction of nitrogen oxides. Those reactions are all markedly accelerated by the presence of a catalyst, so that they can be accomplished during the brief time of passage of the exhaust gases through the converter.
However, the catalytic conversion of pollutants can be accomplished only when the catalyst has reached a sufficient temperature, generally higher than 300 C. As a result, notably on cold starting of an engine, the temperature of the exhaust gases is not sufficient to prime the chemical reactions and a long phase then follows during which the pollutants emitted by the engine are untreated or are not treated sufficiently. It can thus be estimated that 75 to 80% of pollutants are emitted during the first two minutes of running of the engine.
As a result, the presence of a turbine in the path of the exhaust gases upstream of the catalytic converter, which has the effect of appreciably increasing the heat losses undergone by the exhaust gases before they enter the catalytic converter, owing to lengthening of the path crossed by the exhaust gases and to the power supplied to the turbine, extends the priming time of the catalyst and, consequently, increases emissions of pollutants.